1. Technical Field
This invention concerns a fuel distributor for a fuel supply duct, a fuel supply system equipped with the aforementioned fuel distributor, and a combustion system equipped with the aforementioned fuel supply system, and in particular, concerns a fuel distributor for a fuel supply duct that is favorable for improving the combustion characteristics of a brown coal fired boiler.
2. Background Art
FIG. 20 shows an example of a prior art brown coal combustion system for a boiler. The brown coal combustion system and the boiler structure are comprised of a coal hopper 1, a mill 3, which pulverizes the coal supplied from said hopper 1, a fuel supply duct 4, which conveys a mixed fluid made up of the coal particles supplied from said mill 3 and a coal particle carrier gas (hereinafter, the coal particles may be referred to as “pulverized coal” and the mixture of coal particles and coal particle carrier gas may be referred to as “mixed fluid” or “solid-gas two-phase flow”), burners 5, which are connected to the end parts of said fuel supply duct 4, a furnace 8, having burners 5 provided on the side walls thereof, an exhaust gas duct 6, which connects a wall opening of furnace 8 with mill 3 for use of the exhaust gas of the coal particles burnt by said burners 5 as coal particle carrier gas, and a heat exchanger tube 9, which is provided inside said furnace 8.
Lump-form coal A is cut out at a feeder 2, provided at the lower part of hopper 1, and is fed continuously into mill 3. Though a fan mill is used as mill 3 in many cases, the structure of mill 3 is not limited to a fan mill.
At mill 3, the coal is dried by a high-temperature exhaust gas B, which has an oxygen concentration of less than 21% and is introduced from furnace 8 via exhaust gas duct 6, and is pulverized at the same time. The mixed fluid C of coal particles (pulverized coal), obtained by pulverization of granular coal, and exhaust gas is supplied via fuel supply duct 4 to burners 5, which are provided in a plurality of stages in the vertical direction of the side walls of furnace 8. The coal particles supplied to burners 5 are burnt inside furnace 8, thereby forming a flame, and the resulting radiant heat undergoes heat absorption by heat exchanger tube 9 provided at the furnace side walls and the upper part of the furnace and makes steam.
From fuel supply duct 4, the mixed fluid C is distributed among the plurality of stages of burners 5 that are installed on the side walls of furnace 8, and in many cases, burners 5 are arranged in two to four stages. Also, in many cases, these burners 5 of a plurality of stages are provided in the vertical direction of the side walls of furnace 8 for each mill 3 (a plurality of mills are installed for each boiler can). This is because the discharge pressure capacity of fan mill 3 is low in comparison to a normal, centrifugal type turbo blower, etc. That is, the pressure loss at fuel supply duct 4 must be restrained and in order to make fuel supply duct 4 simple and avoid making its length longer than necessary, it is more advantageous to arrange the burner group in the vertical direction than in the horizontal direction.
Next, an example of the method of combustion in the boiler furnace 8 shown in FIG. 20 shall be described.
Though for example when the load of the boiler is low, the amount of coal A supplied to burners 5 is lowered, the flow velocity of the coal particle carrier gas (boiler exhaust gas) in fuel supply duct 4 is kept at a fixed flow velocity so that the flow velocity will not fall below the minimum flow velocity necessary for stable carrying of the coal particles and so as to convey the coal particles, resulting from the pulverization of coal A by mill 3, in a stable manner from mill 3 to burners 5. Thus, when the load of the boiler is low, the concentration of coal particles in the mixed fluid C that is supplied to burners 5 becomes low and the fuel ignition characteristics at burners 5 can become unstable.
As a countermeasure, a part of the plurality of mills 3 is stopped temporarily (mill cutting; e.g. the number of operating mills is changed from four units to two units) and, at the same time, the concentration of coal particles (pulverized coal) in the mixed fluid supplied to the burner 5 of each stage is changed respectively.
The prior art illustrated in FIGS. 27, 28, and 29 are known as art for concentrating the fuel in the fuel supply duct 4 that conveys coal to burners 5. In these fuel concentrating techniques, the concentrations of coal particles supplied to the respective burners at the upper stage side and lower stage side are adjusted.
With the example shown in FIG. 27, a large-diameter main fuel supply duct (main duct) 4 for supplying fuel is provided at the upstream side of the flow path of mixed fluid C, and a small-diameter fuel supply duct (branch duct) 102 is provided at the downstream side of main duct 4. This fuel supply duct (branch duct) 102 is inserted, thereby branching the flow path of mixed fluid C into two ducts, and a lower stage burner 501 and an upper stage burner 502 are connected to the end parts of the respective ducts. With the structure shown in FIG. 27, a conical deflector 105 is installed at the inner part of the large-diameter main duct 4 at the upstream side of the base opening of small-diameter branch duct 102 and the inertial force of the coal particles is used to cause the coal particles to gather towards the inner wall of large-diameter duct 4, thereby making the concentration of coal particles supplied to lower-stage burner 501 higher than the concentration of coal particles supplied to upper-stage burner 502.
With the example shown in FIG. 28, the fuel supply duct (main duct) 4 is branched into three ducts, an upper-stage burner 503, a middle-stage burner 504, and a lower-stage burner 505, which are installed at the ends of the branched branch ducts 107, 108, and 109, respectively. Distributors (dampers) 115 to 117 are installed inside the three branch ducts 107 to 109, respectively, and the respective flow resistances of mixed fluid C in branch ducts 107 to 109 are adjusted by the tilt angles of dampers 115 to 117 to control the flow rate of the mixed fluid.
With the example shown in FIG. 29, a main duct 4, for supplying the fuel conveyed from mill 3, is connected to an upper-stage burner 506 without being changed in cross-sectional area and a branch duct 121, connecting to a lower-stage burner 507, is provided in the middle. This prior art provides the effect that the concentration of coal particles in the mixed fluid C that is supplied to upper-stage burner 506 is increased by the inertial force of the coal particles.
The above-described prior art illustrated in FIG. 27 through FIG. 29 have the problem that with the burners 501 to 507 and the fuel supply ducts connected to the burners 501 to 507, the concentrations of coal in mixed fluid C in the branch ducts connected to main duct 4 cannot be adjusted. Dampers 115 to 117 are installed inside the three branch ducts 107 to 109 respectively as shown in FIG. 28, and though the flow resistance of mixed fluid C, comprised of coal particles and the carrier gas therefore, can be changed within each of branch ducts 107 to 109, it does not enable just the coal particle concentration to be changed selectively.
Also, with the fuel supply duct shown in FIG. 27 and FIG. 29, since members for adjusting the damper and the associated flow path opening are not provided, the concentrations of coal particles in the mixed fluid inside the branch ducts 102 and 121, connected to main duct 4, cannot be changed as suited in accordance to changes in the boiler load.
The above described prior art also have the problem that the distribution of the coal particle concentration in the fuel supply duct (main duct) 4, which supplies mixed fluid C from fan mill 3 to the respective stages of burners 5 in boiler furnace 8, is difficult to adjust.
At main duct 4 in the vicinity of the exit part of fan mill 3, the concentration of coal particles per unit area of the cross unit is not necessarily uniform and there is a distribution of concentration in many cases. This is because the coal particles are introduced into main duct 4 by the centrifugal force of a fan blade 16, which, as shown in FIG. 21, is disposed inside fan mill 3 and is rotated at high speed. FIG. 21 shows the flow conditions of coal in fan mill 3, and the coal that is supplied to fan mill 3 is pulverized finely by the collision with fan blade 16 and the coal particles are pushed towards the inner wall side of housing 17 of fan mill 3 by the centrifugal force resulting from the rotation of fan blade 16. As a result, this gives rise to a bias in the coal particle concentration of the mixed fluid, which is comprised of a solid-gas two-phase flow, at main duct 4 in the vicinity of the exit part of fan mill 3, and a flow d, having a high concentration of coal particles, and a flow d′, having a non-high concentration of coal particles, are formed in the cross-sectional direction of main duct 4 (this may be referred to hereinafter as the “bias of the solid-gas two-phase flow”).
The centrifugal force of fan blade 16 is mainly determined by the installation position of fan mill 3, the structure of fuel supply duct 4, etc., and it is difficult to ascertain the coal particle concentration distribution in accordance to the differences in the structures of fan mill 3 and burners 5 prior to operation of the coal combustion system.
Also, in the case where a classifier 18, such as shown in FIG. 22, is installed in main duct 4 at the exit part of fan mill 3 in order to make fine the grain size of the coal particles that are conveyed to burners 5 of boiler furnace 8, the above-described bias of the solid-gas two-phase flow strengthens within the main duct 4 that is connected to the downstream part of classifier 18. This action shall now be described using FIG. 22.
The solid-gas two-phase flows d and d′ that have been conveyed from fan mill 3 via the main duct 4 at the upstream side of classifier 18 collide with the collision plate 21 provided on classifier 18, and thereafter the rough coal particles f drop in the direction of the entrance of fan mill 3 and are returned to the entrance of the unillustrated fan mill 3 via duct 20. Meanwhile, the fine coal particles e are supplied to the respective burner-stages of furnace 8 via main duct 4 at the downstream side of distributor 18. In this process, the fine coal particles e inside main duct 4 drift, due to inertial force, in the direction of the wall of main duct 4 that is closer to the inner wall of classifier housing 19 that opposes the inner wall of housing 19 at the side at which collision plate 21 of classifier 18 is installed, and a large non-uniformity thus forms in the distribution of the coal particle concentration in the direction of the cross section of main duct 4.
If mixed fluid C is conveyed into each of the branch ducts that branch from main duct 4 with the above-described non-uniformity of coal particle concentration distribution being maintained, coal particle fuel of a suitable concentration may not be supplied to each burner 5. For example, a mixed fluid C of low coal particle concentration may be conveyed to a burner 5 to which a mixed fluid C of high coal particle concentration should be conveyed. Especially in the case where a boiler is to be operated at low load, if a mixed fluid C of low coal particle concentration is conveyed to a burner 5 to which a mixed fluid C of high coal particle concentration should be conveyed, the combustion condition of the flame can become unstable and cause a flame-out.
When a boiler is to be operated at low load, the mill load must be lowered, and though the supply amount of coal is lowered accordingly, the flow rate of the coal carrier gas cannot be lowered below a predetermined flow rate (minimum flow rate) for stable conveying of the coal particles. Thus in order to prevent flame-out, the concentration of coal particles in the mixed fluid C, which is to be supplied to a specific burner among the burners 5 disposed in a plurality of stages in the furnace, must be thickened to secure stability of ignition and the stable combustion of the flame at burner 5.
Furthermore, in the case where brown coal or other coal that contains a high amount of water or ash is used as the boiler fuel, the coal particle concentration range, in which a stable burner flame can be maintained, is determined in accordance to the proportion of water or ash contained in the coal in the actual operation of the boiler.
Also, the stability of the flame of burner 5 is strongly dependent on the coal particle concentration, water concentration, and ash concentration supplied to burner 5, and it is known by experience that the stability of the burner flame is better the higher the coal particle concentration, the lower the water concentration, and the lower the ash concentration. Since coal, such as brown coal, contains a high amount of water or ash, the securing of the stability of the burner flame will be important in the case where brown coal is used as fuel.
FIG. 23 and FIG. 26 show an example of mill cutting (from four units to two units) for low load operation of a furnace 8 provided with burners 5 at the corner parts of opposing walls. FIG. 26 shows the burner flame conditions when the load is even lower than that in the case of FIG. 23. When mill cutting is carried out for low load operation of the boiler and the thermal load within furnace 8 decreases, a stable, high-temperature combustion zone will not be formed at the central part of furnace 8 as shown in FIG. 23 and FIG. 26, and the method of achieving stable combustion by self flame stabilization at each burner is carried out. In this case, unless the coal particle concentration is not adjusted appropriately, the combustion of coal becomes unstable and the stable operation of the boiler is made difficult.
Generally when the boiler load is low, the concentration of coal particles supplied to the burners of specific stages, among the plurality of stages of burners disposed in the vertical direction of the furnace side wall, is increased to stabilize the burner flame combustion at these specific stages and the stability of combustion of the furnace as a whole is thereby secured. However, even if high amounts of concentrated coal particles are supplied to burners of specific stages and the burner ignition stability is improved, the exhaust gas temperature at the furnace exit decreases due to the relationship between heat absorption by the furnace walls in the furnace height direction and the flame temperature distribution within the furnace, thereby preventing the obtaining of the predetermined steam temperature. For ignition stability of the coal and for making the temperature of the furnace exit exhaust gas the predetermined temperature, the adjustment of the concentrations of coal particles supplied to the respective burners 5 disposed in the upper and lower stages becomes important.
An object of this invention is to provide a fuel distributor for a fuel supply duct, by which solid fuel can be supplied to a burner in a manner whereby ignition stability and stable combustion of the ignited flame can be achieved even when the load of a boiler is low; a fuel supply system that is equipped with the aforementioned fuel distributor for a fuel supply duct; and a fuel combustion device that is equipped with the aforementioned fuel supply system.
Another object of this invention is to provide a fuel distributor for a fuel supply duct, which is equipped with the function of deflecting solid fuel of high concentration in a mixed fluid, comprised of the solid fuel and carrier gas, in an intended direction; a fuel supply system that is equipped with the aforementioned fuel distributor for a fuel supply duct; and a fuel combustion device that is equipped with the aforementioned fuel supply system.
Also, generally during full load (100% load) operation of a boiler, an example of which is shown in FIG. 20, the temperature of the gas exiting the boiler furnace is set so that after the gas undergoes heat absorption by heat exchanger walls, which are installed along the gas flow path at the downstream side of the exit of furnace 8, and by a heat exchanger tube 9, which is installed inside the abovementioned gas flow path, and reaches an unillustrated posterior heat exchanger part of the furnace, the gas temperature will be lower than the melting point of the ash contained in the gas. The temperature of the gas exiting the boiler furnace during full load operation of the boiler is also set so that the metal temperature of the surface of an unillustrated heat exchanger tube installed at the above-mentioned posterior heat exchanger part will not be raised excessively to or above the heat resistant temperature of the surface.
However, when the boiler undergoes the transition from full load operation to partial load operation, sine the amount of heat input into furnace 8 decreases, the gas temperature at the boiler furnace exit decreases and the steam temperature at the boiler exit falls below the steam temperature required at the turbine entrance at the steam demanding end (this temperature may also be referred to as the “steam temperature required at the demanding end”).
Thus another object of this invention is to provide a fuel distributor for a fuel supply duct, with which when a boiler that uses a mixed fluid, comprised of solid fuel and a carrier gas therefore, is switched from full load operation to partial low operation, the temperature of the gas at the boiler furnace exit is prevented from dropping excessively so that the steam temperature at the boiler exit will not become less than or equal to the steam temperature required at the demanding end, and a method of operating a boiler equipped with the said fuel distributor for fuel supply duct.